The present invention relates generally to continuous air monitors, and more particularly to the reduction in background in the measurement of radioactive emission activity in aerosol particles.
The presence of aerosol particles which emit alpha, beta, gamma, and x-ray radiations (e.g., aerosol particles containing transuranic (TRU) elements) is detected through use of continuous air monitors (CAMs) in the workplace or in stacks. Typically, these devices collect aerosol particles on a substrate (e.g., a filter or an inertial impaction collection element) and detect the radioactive emissions (generally alpha emissions in the case of TRUs) employing a planar solid state detector placed parallel to the collector with a gap of approximately 5 mm between the detector and the collector. In the case of a filter collector, which is the most common collection element, the aerosol is drawn into the gap between the filter and detector and then passes through the filter where particle collection occurs In general, as the radionuclides associated with a collected aerosol sample decay, the alpha particles or other radiations which leave the collected aerosol particles in the solid angle of the detector/filter configuration are registered by the detector. It is common to employ a detector which provides a measurement of the kinetic energy of the alpha particles or other radiations, but in some devices only a gross count of activity is obtained. In a perfect environment (e.g., a vacuum chamber sample holder with the sample being a monomolecular layer on a collection substrate) the energy of any decay event of a given isotope would be registered as a constant. In turn, this suggests that through use of a multichannel analyzer (MCA), accumulation of the counts associated with a given energy channel would provide a measurement of the concentration of a particular isotope in the aerosol state. However, the CAM environment is not ideal, and there are several factors which create problems in determining the concentration of radionuclides in the aerosol state from data produced by the detector/MCA, particularly for the alpha particle radiation energy. Included are: 1) the presence of air in the gap between collector and detector which causes a broadening of the energy spectra, 2) interference from particulate matter on the collector, 3) inadvertent loss of particulate matter on internal surfaces of the CAM, and 4) interference in the spectral distribution caused by background alpha-emitters. It is the latter factor which often causes the greatest problems.
There have been many reports concerning the discrete (noncontinuous) measurement of radon progeny both attached to aerosols and unattached in chosen air samples. For example, in "An Evaluation Of Unattached Radon (And Thorium) Daughter Measurement Techniques," by Antoon W. Van Der Vooren, Anthony Busigin, and Colin R. Phillips, Health Physics 42, 801 (1982), the authors evaluate collection efficiencies of parallel plate, inertial impactor, and wire screen devices used for measurement of the unattached fraction of radon (or thoron) daughters for collection of the attached fraction of the aerosol. In "Measurement Of Charged And Unattached Fractions Of Radon And Thoron Daughters In Two Canadian Uranium Mines," by Charlett J. Busigin, Anthony Busigin, and Colin R. Phillips, Health Physics 44, 165 (1983), the authors utilize a cylindrical condenser to perform charged fraction measurements. Additionally, in "Experimental Measurements Of The Diffusion Coefficient Of .sup.212 Pb," by Y. F. Su, G. J. Newton, Y. S. Cheng, and H. C. Yeh, Health Physics 56, 309 (1989), the authors use a cylindrical diffusion tube and screens to measure deposition of radon progeny. None of these references teach the use of the apparatus employed therein for reducing the background of radon progeny for the purpose of making reliable aerosol radioactive emission activity determinations in ambient air in a continuous air monitor.
By contrast however, aerosol impactor devices have been used both for size characterization and analysis, and as a procedure for fractionating radon progeny from alpha emitting transuranics, thereby achieving background suppression. In "An On-Line Monitor For Alpha-Emitting Aerosols," by Thomas J. Yule, IEEE Transactions On Nuclear Science NS-25, 762 (1978), Yule reported the development of a CAM sampler utilizing a virtual impaction concept to separate a fine fraction of an aerosol sample from a coarse fraction. The fine fraction is discarded in the exhaust of the device without counting. A high sampling rate (10 CFM) is needed in this device to acquire a usable sample, since only 7% of the flow is actually filtered and counted. Two stages of virtual impaction are used to achieve a reported 50% cut point of about 2 .mu.m. The disadvantage of this device include: a) a sizeable percentage of the TRU sample is lost along with the fine fraction containing the radon daughters that is discarded, with an attendant loss of sensitivity; b) the 10 CFM sampling rate requirement imposed by the inefficiency of collection cannot be supported in most large plutonium handling facilities where dozens to hundreds of CAMs must be operated from house vacuum; c) the additional costs and complexity of building and maintaining a two-stage virtual impactor offsets the gain in performance; d) operation of the second stage in the coarse fraction exhaust of the first stage leads to inadvertent losses of particles; and e) without any other means for removing radon daughters, the pass-through flow contains an extra burden of radioactive materials.
Another version of an impactor CAM device utilizes a large, high volume annular impactor to selectively deposit coarse size particulates on a grease-coated planchet directly below the jet. Since this is a high volume sampling device (&gt;10 CFM), it has been difficult to apply widely (See, e.g., "A Continuous Monitor For Prompt Detection Of Airborne Plutonium," by J. M. Alexander, Health Physics 12, 553 (1966)). A disadvantage of these devices is that typically, the detection principle is gross alpha counting of the sample planchet which does not yield energy distribution data. Hence, no further data processing to better refine the transuranic content estimate is possible. This can be a problem in some applications where the large particles carry radon progeny with them to the collection substrate. Another disadvantage of this type of impactor is that while it can fractionate an aerosol at a somewhat lower cut point than a virtual impactor, it is subject to solid particle reentrainment problems. And finally, these devices are notorious for producing a nonhomogeneous deposit on the collector which under some circumstances can reduce the sensitivity of the detection process.
A third related approach based on impaction is found in a CAM device described in "The ZPR-9 Airborne Plutonium Monitoring System," by Gordon K. Rusch and William P. McDowell, IEEE Transactions On Nuclear Science NS-23, 690 (1976). This is a modification of the Alexander, supra design. A large volume of sampled air is directed through an annular jet to an impactor surface where large particles are deposited. The majority of the flow is diverted through 180.degree. and contains the radon progeny and the fine fraction of the plutonium aerosol which is discharged from the apparatus uncounted. The impactor surface is a passivated, diffused-junction silicon solid-state alpha detector. Such an apparatus permits the direct detection of plutonium and background alpha emissions with an energy-discriminating detector. In addition to background suppression by removal of a fraction of the radon progeny in the fine fraction, additional signal and data processing can then be employed to yield further background compensation. A thin grease coating applied daily to the surface of the detector reduces particle bounce and reentrainment. Disadvantages of this device are that a large sampling rate is required, a large fraction of the plutonium sample is discarded uncounted, and energy resolution of the detector deteriorates in time due to accumulated deposits on the detector and the grease coating.
Accordingly, it is an object of the present invention to provide an apparatus having a reduced background for determining the radioactive emission activity of aerosols which fractionates high-mobility radon progeny without an unacceptable loss of larger particles, such that virtually the entire sample, minus substantially all of the high-mobility radon progeny, is collected and counted.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.